Method and system for high speed measuring of microscopic targets

ABSTRACT

A system including confocal and triangulation-based scanners or subsystems provides data which is both acquired and processed under the control of a control algorithm to obtain information such as dimensional information about microscopic targets which may be “non-cooperative.” The “non-cooperative” targets are illuminated with a scanning beam of electromagnetic radiation such as laser light incident from a first direction. A confocal detector of the electromagnetic radiation is placed at a first location for receiving reflected radiation which is substantially optically collinear with the incident beam of electromagnetic radiation. The system includes a spatial filter for attenuating background energy. The triangulation-based subsystem also includes a detector of electromagnetic radiation which is placed at a second location which is non-collinear with respect to the incident beam. This detector has a position sensitive axis. Digital data is derived from signals produced by the detectors. In this way, data from at least one triangulation-based channel is acquired in parallel or sequentially with at least one slice of confocal image data having substantially perfect temporal and spatial registration with the triangulation-based sensor data. This allows for fusion or further processing of the data for use with a predetermined measurement algorithm to thereby obtain information about the targets.

CROSS-REFERENCE TO RELATED APPLICATION

“This is a divisional of copending application Ser. No. 09/035,564 filedMar. 5, 1998.”

This application is related to U.S. patent application entitled“Versatile Method and System for High Speed, 3D Imaging of MicroscopicTargets” filed on the same day as this application.

TECHNICAL FIELD

This invention relates to methods and systems for high speed measuringof targets and, in particular, to methods and systems for high speedmeasuring of microscopic targets which may be “non-cooperative.”

BACKGROUND ART

Recognition of Need

A class of three-dimensional imaging and measurement applications nowrequires unprecedented demonstration of capability to support newmicroelectronic and micromechanical fabrication technologies. Forexample, emerging semiconductor fabrication technologies are directedtoward establishing a high density of interconnection between the chipand package. The “bumped wafer” and miniature ball grid array (“μ-BGA”)markets are emerging, and large scale growth is predicted. For instance,NEMI (National Electronics Manufacturing Initiative) has clearlyindicated that the miniature array technologies are to replacetraditional wire bonding interconnects. Manufacturers are experimentingwith new processes. Measurement tools to support their efforts willrequire versatility.

For example, “dummy wafers” are used for many experiments, which have aspecular and featureless surface onto which interconnects are placed.The appearance is much different than patterned wafers seen in typicalproduction environments. This imaging phenomena is of little concern tothe process engineer. In fact, the most difficult imaging problems maycoincide with the best choice of process. Industry process developmentengineers indicate that reflowed spherical solder bumps with a smoothsurface finish, sometimes a nearly perfect mirror, may be the preferredtechnology for the chip interconnects. The surface reflectance will varybecause of process engineers' choices of relative content of lead andtin. Such targets are often “uncooperative.”

The chips onto which the balls are placed are subsequently attached toprinted circuit boards where both flattened and spherical matinginterconnects can be expected, with either a dull or smooth surfacefinish. All combinations are expected. Other geometric shapes (wire withflat top, cones) can be expected in the future which will posemeasurement challenges, particularly when the surface is specular withspherical or cylindrical geometry, including concavities.

Such “non-cooperative” targets, ( i.e. those which present challengesfor measurement systems as a result of light reflection, scattering, andgeometry), are and will continue to be growing in occurrence forsemiconductor, micromachining, and mass storage imaging applications. Aspecific growing need is recognized for an imaging system capable ofimproving dimensional measurement of μ-BGAs and bumped wafers (i.e.“spherical mirrors” on variable wafer backgrounds) and other suchtargets, which are “non-cooperative” with respect to traditional imagingsystems. As inspection and measurement requirements for industriesrequiring microscopic measurement capabilities, for instancesemiconductor and mass storage, become more demanding, extraordinaryversatility will be needed for handling wide variation in scale, targetgeometry, and reflectivity. Similarly, inspection and measurement ofcircuit boards and the dielectric and conductive materials requires aversatile imaging system, particularly for fine geometries and denselypopulated component boards.

As mentioned previously, imaging requirements for the semiconductorpackaging industry include defect detection as part of Package VisualInspection (PVI) measurement of μ-BGA height, coplanarity, diameter, andwafer defects. High resolution and image clarity obtained from reductionof image artifacts are both required for adequate processcharacterization. Problems similar to those in the semiconductor areaare also present when measuring other miniature parts like micromachined(micromechanical) assemblies, like miniature gears and machines, andcomponents utilized in the mass storage industry, including substrates,disk heads, and flexures.

For example, as illustrated in FIGS. 1 and 7, inspection of a very finesolder bump or ball 20 with a “pin” or tip 22 necking down to about 1-3μm in dimension mounted on a solder pad 24, poses a measurement problem.Manufacturers often examine the tip 22 with an electron microscope forinitial evaluation, but such a tool is much too slow for detailedprocess characterization or real time control.

Also, detection of small “hairline” burrs on IC leads is oftensuccessful using gray or 3D data using only triangulation, but falsealarms are common because background noise and reflection from acontainer, such as a tray wall 26, can appear similar to the defect suchas a burr 27, as illustrated in FIG. 2. Conversely, IC leads 28 of an ICchip 30 may be indistinguishable from the background noise 32. Thesefalse alarms are unacceptable and lower yields, thereby decreasing thevalue of inspection equipment.

μ-BGA inspection can be roughly equivalent to measuring a tiny“spherical mirror” (solder ball) mounted on a plane “mirror” (wafer)background; yet, in other cases, where the wafer is patterned and theball has a lower tin content, is a completely different imaging problem.Solutions to such measurement problems will require versatility forhandling the geometric shape and reflectance variation.

Hence, with wafer scale and other sub-micron measurement tasks, thechallenges with material properties will grow, not diminish. There is aneed to measure substrates, conductors, and thickness of films, or thegeometry of micromechanical assemblies such as miniature gears havingdeep, narrow dimensions and varying optical properties, includingpartially transparent layers.

Prior Art Technology

Early work on defect detection of features having specular componentsusing camera-based inspection is described in U.S. Pat. No. 5,058,178and the references cited therein. The method is primarily directedtoward lighting and image processing methods for defect detection ofbumped wafers. The lighting system included combinations of bright anddark field illumination. Measurement of the diameter can be done with acamera system and appropriate illumination, but accuracy is oftenlimited by light scattering and limited depth of focus when highmagnification is required. However, in addition to defect detection andbump presence, there is a need to measure the three dimensional geometryof the bumps for process characterization. The bumps must be coplanar toprovide a proper connection, and the diameter within tolerance for agood connection with the bonding pads.

Triangulation is the most commonly used 3D imaging method and offers agood figure of merit for resolution and speed. U.S. Pat. Nos. 5,024,529and 5,546,189 describe the use of triangulation-based systems forinspection of many industrial parts, including shiny surfaces like pinsof a grid array. U.S. Pat. No. 5,617,209 shows an efficient scanningmethod for grid arrays which has additional benefits for improvingaccuracy. The method of using an angled beam of radiant energy can beused for triangulation, confocal or general line scan systems.Unfortunately, triangulation systems are not immune to fundamentallimitations like occlusion and sensitivity to background reflection.Furthermore, at high magnification, the depth of focus can limitperformance of systems, particularly edge location accuracy, when theobject has substantial relief and a wide dynamic range (i.e. variationin surface reflectance). In some cases, camera-based systems have beencombined with triangulation systems to enhance measurement capability asdisclosed in the publication entitled “Automatic Inspection of ComponentBoards Using 3D and Grey Scale Vision” by D. Svetkoff et al.,PROCEEDINGS INTERNATIONAL SYMPOSIUM ON MICROELECTRONICS, 1986.

Confocal imaging, as originally disclosed by Minsky in U.S. Pat. No.3,013,467, and publications: (1) “Dynamic Focusing in the ConfocalScanning Microscope” by T. Wilson et al.; (2) “Digital Image Processingof Confocal Images” by I. J. Cox and C. J. R. Sheppard; (3)“Three-Dimensional Surface Measurement Using the Confocal SensingMicroscope” by D. K. Hamilton and T. Wilson; (4) “Scanning OpticalMicroscope Incorporating a Digital Framestore and Microcomputer” by I.J. Cox and C. J. R. Sheppard; and (5) “Depth of Field in the ScanningMicroscope” by C. J. R. Sheppard and T. Wilson, is similar tocomputerized tomography where slices in depth are sequentially acquiredand the data is used to “reconstruct” a light scattering volume. Inprinciple, an image is always formed of an object at a focal plane astaught in elementary physics, but over a region of depth there are aninfinite number of planes which are out of focus yet return energy. Thatis to say that the lens equation for image formation is based on anidealization of an “object plane” and “image plane”.

In the case of conventional confocal imaging, the slices are determinedfrom the in-focus plane, and out-of-focus light (in front and back ofthe focal plane) is strongly attenuated with a pinhole or slit. Typicalconfocal systems use fine increments for axial positioning for bestdiscrimination between adjacent layers in depth, for example,semi-transparent biological samples. However, the method need not berestricted to the traditional transparent or translucent objects, butcan be applied both as a depth measurement tool and image enhancementmethod using reflected light for contrast improvement through straylight rejection. As with any method, there are advantages anddisadvantages.

Application of confocal imaging to semiconductor measurement isdisclosed in U.S. Pat. Nos. 4,689,491, 5,479,252 and 5,248,876.Operation of several confocal systems is described in U.S. Pat. Nos.4,827,125; 4,863,226; 4,893,008; 5,153,428; 5,381,236; 5,510,894;5,594,235; and 5,483,055 and H 1,530. Much of the recent work isdirected toward improvements, resulting in reduction of the image memorystorage requirements (store maximum, not volume), improving theefficiency and fine positioning capability of autofocus systems(coarse/fine search), exposure control for improved dynamic range, andsome image enhancement methods.

Similarly, variations in confocal acquisition methods are taught in theart to solve specific problems or optimize designs for specificapplications as taught in U.S. Pat. Nos. 5,239,178 and 4,873,653.However, present confocal systems are constrained by sequential slicingof the volume, whereas triangulation systems detect the top surface ofthe volume (profile) directly resulting in much higher speed.

In U.S. Pat. No. 5,448,359 such speed limitations are partiallycircumvented by utilizing a plurality of detectors and spatial filtersin the confocal receiver optical path. A circuit to locate the detectorproducing maximum intensity is disclosed.

Similarly, USSR patent document No. 868,341 discloses a plurality ofdetectors with apertures (confocal) and electronic circuitry to obtainfocus (3D) information about objects. The intensity of each detector iscompared and used to adjust the position of the imaging system along theoptical axis so as to clear the mismatch. In each case, a tradeoff isdetermined between depth sensitivity, complexity, and measurement speed.

Other approaches to imaging of “non-cooperative” targets, many directedtoward solder joint inspection, have been proposed to measure depth orfillet shape. These are described in the U.S. patent to Chen et al. U.S.Pat. No. 5,118,192 and a Nagoya solder joint inspection system describedin “NLB Laser Inspector—NLB-7700M Specifications” by Nagoya ElectricWorks Co., Ltd. 1994. The system uses specularly reflected light toexamine the shape of solder fillets, and to determine presence/absenceof solder. FIG. E in Section 6 thereof shows a missing fillet and thesignals received from a plurality of detectors. A detector 6 correspondsto an “on-axis” detector, and the information is useful for estimatingthe diameter of the solder bump. For instance, the detector 6 receives alarge signal near the top of the ball, a weak signal from the curvededge, and typically a strong signal from the area adjacent to the bump.However, narrow angle multiple reflections from the edge of the ball cancorrupt the measurement and result in ambiguous edge locations.Furthermore, the sensitivity of the system may not be adequate todetermine the height of regions which do not have a substantial specularreflection component.

Similarly, a recent version of the IPK solder joint inspection systemmanufactured by Panasert includes a coaxial detector with atriangulation-based sensing system as illustrated in their brochureentitled “IPK-V” believed to be published in 1997. The μ-BGA, bumpeddie, and numerous other problems range from scenarios where prior arttechnology is adequate, but in many cases unacceptable, and eveninoperable conditions exist.

Wafer measurement and defect detection systems have utilized multipledetectors advantageously. U.S. Pat. No. 5,416,594 describes a systemwhich uses both reflected and scattered light for detection of defectsand thin film measurements. The reflected beam is received at an angleof reflection which is non-collinear with the transmitted beam and thescattered light is collected over a relatively large angle whichexcludes the reflected beam energy. The scattered light beam,representative of surface defects, may be collected at an angle which iswidely separated (more than 30 deg.) from the incident beam. Theoff-axis illumination and the corresponding reflected beam are utilizedfor film thickness measurements, sometimes with multiple laserwavelengths. The scattered light signal is analyzed in conjunction withthat representing the reflected light. Although the imaging geometry iswell matched to the specific cited inspection requirements, there areseveral potential disadvantages encountered when attempting tosimultaneously provide information about surface defects and say, thepeak height of interconnects like solder bumps (which have substantialheight) and the corresponding diameter and shape.

Commercial success has not been widespread, although many approacheshave been proposed. Hence, there is a need for a system and method forthree-dimensional imaging capable of performing with both “cooperative”and “non-cooperative” targets. To be useful, the method and system mustbe accurate, robust, and have high measurement speed, the latter being atraditional limit to the use of widespread confocal imaging.

SUMMARY OF THE INVENTION

A method of the present invention overcomes the limitations of the priorart imaging of non-cooperative targets by illuminating a surface with ascanning beam, acquiring data from at least one triangulation-basedchannel, and acquiring in parallel or sequentially at least one slice ofconfocal image data having substantially perfect temporal and spatialregistration with the triangulation-based sensor data, allowing forfusion or processing of the data for use with a predeterminedmeasurement algorithm.

The objects of a system of the present invention are met by utilizing acombination of confocal and triangulation-based data acquisition, with acontrol algorithm guiding the cooperative data acquisition andsubsequent processing.

The invention is a method and system for developing three-dimensionalinformation about an object by illuminating an object with a focusedbeam of electromagnetic radiation incident from a first direction. Adetector of electromagnetic radiation is placed at a first location forreceiving reflected radiation which is substantially optically collinearwith the incident beam of electromagnetic radiation, and the detectionsystem includes a spatial filter for attenuating background energy.Another detector of electromagnetic radiation is placed at a secondlocation which is non-collinear with respect to the incident beam. Thedetector has a position sensitive axis. Digital data is derived fromsignals produced by said first and second detectors. The digital data isthen processed to generate information about the object.

Specific objects of the invention include:

An object of the invention is to provide an integrated method and systemfor high speed measuring to obtain measurements for conductor traces(height˜1-3 μm) and/or interconnects (i.e. 10-300 μm bumps) onsemiconductor devices.

An object of the invention is to provide a method and system for highspeed measuring which has diverse measurement and defect detectioncapability with a combination of a confocal sensor and triangulationallowing for measurement of miniature, complex geometry present in themicroelectronics, micromechanical, and disk storage industries.

An object of the invention is to provide a method and system for highspeed measuring to obtain information from either of two channels usedto guide subsequent data acquisition operations in either or bothchannels. For example, sparse data may be acquired with atriangulation-based system at high speed, and the information used toguide the high speed selection of confocal slices, perhaps in windowedregions. FIG. 3 illustrates imaging geometry of a solder ball 29 (i.e.spherical mirror) of radius R (i.e. R<150 μm typically) formed on a pad31.

An object of the invention is to provide a high speed method and systemfor measuring which can obtain reasonable height estimates of the bumpsor “spherical mirrors” in a “pre-screening” operation and locatedefective bumps or wafers at high speed. The results would define therange for additional slices (i.e. if needed) for precise verification ofthe geometry of regions passing the “prescreening” test. Therefore,maintaining wafer inspection times will remain as minutes, not hours.For “sparse” patterns, “windowing” could increase the speed ofmeasurement for localized regions. FIGS. 4a and 4 b are top schematicimages of specular solder balls 34 (indicated by phantom lines in FIG.4a) using triangulation and confocal imaging, respectively, inaccordance with the present invention. The balls 34 of FIG. 4b havespecular ball tips 35 formed on pads 36 which, in turn, are located on ashiny “dummy” wafer 38. The 3D image of FIG. 4a (i.e. including specularreflections from regions 35′ of the ball 34 adjacent the ball tips) isformed by a triangulation-based system having dual detectors to provideZ measurement, bump presence and defect information. The confocal sliceimage of FIG. 4b provides diameter, Z measurement and defectinformation. In both FIGS. 4a and 4 b, a flat bump having diffusereflection is indicated at 40, an empty pad (i.e. missing bump) isindicated at 42, and a smashed bump (i.e. defect) is indicated at 44.

Referring specifically to FIG. 5, an object of the invention is toprovide a high speed method and system for measuring a miniaturespherical mirror like a solder ball 46 or wafer, mounted on a planemirror or pad 48 formed on a substrate 50 and producing a very highcontrast bump-background image allowing for accurate measurement ofdiameter, devoid of occlusion and with minimal reflection noise for manypad backgrounds. FIG. 5 shows a spatial filter 52 through which anincident ray 54 passes and bounces off the ball surface to formreflected rays 56, multiple reflections 58, and specular reflection 60.The spatial filter 52 (i.e. confocal slit) provides the indicatedfiltering action.

An object of the invention is to provide a high speed method and systemfor measuring which have significant advantages over conventional cameraand lighting systems, even with relatively few slices of spatiallyfiltered data.

An object of the invention is to provide a high speed method and systemfor measuring to, in turn, provide gray scale contrast improvement ofthe image of FIG. 2 for possible detection of defects and reduction offalse “accepts” and “rejects” (i.e. “error” region 33) in any number ofapplications through stray light rejection. One such classification ofburrs 27 and similar defects, like those specified for electronicPackage Visual Inspection (PVI), may be satisfied with this method andsystem and would otherwise be difficult. FIG. 6 is a confocal slice ofthe IC chip 30 of FIG. 2 located in the tray 26 of FIG. 2. FIG. 6illustrates the effect of spatial filtering with best focus near thenominal pad and burr locations. With the present invention, the data ofFIG. 6 is combined with 3D triangulation data for improvedclassification. Also, visualization and measurement of small bumps andpits could be improved. Furthermore, discrimination of edges which isdifficult in the presence of multiple reflection is provided herein.

An object of the invention is to provide a high speed method and systemfor measuring to, in turn, overcome limits of triangulation-basedimaging for “mirrored” wafer backgrounds, where triangulation oftenrequires photon limited detection or nearly so, and to provide afocus-based depth measurement method and system which operates at highspeed.

An object of the invention is to provide a method and system formeasuring at high speed for measurement of ball bumps, and rigid wireinterconnects within the semiconductor industry.

Referring again to FIGS. 1 and 7, an object of the invention is toprovide a method and system for high speed measuring of objects havingcomplex geometry, for instance “ball bumps” 20 and rigid wires 22, withthe speed advantages of a triangulation/confocal combination whileovercoming “enclosed energy” limitations and resulting corruption of the“signal” by optical noise from reflection of the sidelobe energy to thebackground (as illustrated in FIG. 7), producing false readings intriangulation-based systems. In this case, a confocal channel produces ahigher optical signal-to-noise and background rejection, while atriangulation-based system rapidly measures the other features, albeitat least two passes might be required because of the pin height relativeto the necessarily restricted depth of focus.

An object of the invention is to provide an integrated method and systemfor high speed measuring having substantially perfect temporal andspatial registration between two sensors or subsystems of the systemwhich allows “fusion” of the data, with selection of the best sensordata based upon reflectance and contrast, perhaps on a pixel-by-pixelbasis.

An object of the invention is to provide a versatile method and systemfor high speed measuring of targets on the wafer scale for inspectionand measurement. At such higher magnification, material properties varygreatly, from translucent to opaque, and “mirror-like” to matte.

An object of the invention is to provide a method and system for highspeed measuring to provide improved discrimination of metallic surfacesfrom the translucent backgrounds, and to measure materials such asconductive epoxy used for interconnects. Some applications in theoptical storage industry may be best solved with this type of technology(flexure measurement) and at higher magnification (high contrast, diskhead measurement).

An object of the invention is to provide a method and system for highspeed measuring which introduces a feature of increased gray scalecontrast and fidelity from the region of the beam waist, through atleast rudimentary “depth-through-focus detection” capability. At veryhigh magnification, a confocal channel either “competes” or “cooperates”with dual-detector triangulation for the best imaging mode.

An object of the invention is to provide an integrated method and systemfor high speed measuring which can include both high N.A. (numericalaperture) optics and lower N.A. for use with either confocal ortriangulation channels realized with wavelength, time or spatialmultiplexing methods, as illustrated in FIG. 8.

An object of the invention is to provide a method and system for highspeed measuring which provides selectable lateral and depth resolutionfor confocal and triangulation-based imaging through the use ofmultiplexing and programmable or selectable height resolution.

An object of the invention is to provide a method and system for highspeed measuring including high grey scale resolution and dynamic range(sufficient to avoid automatic gain or light control requirements) andprocessing with smoothing algorithms. The smoothing algorithms may beadapted to include known information regarding the physicalcharacteristics of the object.

An object of the present invention is to provide a method and system forhigh speed measuring by obtaining confocal and/or triangulation datarapidly. Objects which are reflective, such as solder joints, substratesand wafers are substantially opaque in a homogeneous medium such as air,unlike several objects traditionally “sliced” with the confocaltechnique. As such, an object of the invention is to estimate the depthof reflective objects using estimation techniques and relatively fewslices compared to traditional “peak detection” systems utilized forconfocal imaging. The smoothing and estimation techniques could beutilized with a single confocal detector when data is acquired withaxial translation, with multiple detectors involving no translation or acombination of the two.

A further object of the present invention is to provide a method andsystem for high speed measuring by adapting smoothing and/or estimationalgorithms based upon a priori information regarding the physicalcharacteristics of objects within a region of interest, thereby avoidingcorruption of the measurements associated with peak search methods.

A further object of the invention is to provide measurement capabilityof both “featureless” and textured surfaces using an appropriateselection of information.

An object of the invention is to provide an improved method of measuringusing confocal imaging, used alone or in combination with triangulation,where acquisition times are reduced with the use of a solid state beamdeflector having retrace times on the order of 1-10 microseconds,whereby pixel rates well in excess of video rates are achievable.

A further object of the invention is to provide an improved method ofmeasuring using confocal imaging where mechanical motion requirementsfor axial translation of the position of focus of the illumination beamis reduced or eliminated.

In carrying out the above objects and other objects of the presentinvention, a method is provided for developing dimensional informationabout an object on a specular background utilizing a scanning systemhaving a sensor. The scanning system scans an illumination beam ofelectromagnetic energy. The method includes the step of determiningreference data based on an illumination beam reflected from the specularbackground. The method further includes the step of positioning thesensor based on the reference data so that a waist of the illuminationbeam substantially coincides with an expected predetermined 3D locationof the object so as to enhance contrast and obtain three-dimensionalsensor data and/or confocal sensor data. The method finally includes thestep of processing the sensor data to obtain the dimensionalinformation.

Still further in carrying out the above objects and other objects of thepresent invention, a system is provided for developing dimensionalinformation about an object. The confocal system includes at least oneilluminator for illuminating the object with at least one beam ofelectromagnetic energy to obtain at least one reflected beam ofelectromagnetic energy, a confocal detector for detecting the at leastone reflected beam of electromagnetic energy and producing at least onesignal, a signal processor for processing the at least one signal toobtain confocal data and a data processor having digital data processingdata smoothing and curve fitting algorithms for processing the confocaldata with a priori knowledge about the object to obtain the dimensionalinformation whereby the accuracy of the confocal data is improved (i.e.,particularly with the use of fewer slices acquired at relatively coarseincrements with respect to the attainable height resolution).

Further in carrying out the above objects and other objects of thepresent invention, a method is provided for inspecting bumps on a wafer.The method includes the steps of acquiring reference data based on 3Dinformation obtained from either a confocal subsystem or a triangulationsubsystem having a triangulation sensor. The method further includes thestep of generating a scan based upon the reference data to obtain 3Ddata wherein the 3D data is obtained from the triangulation sensor. Themethod finally includes the step of determining height of the bumpsbased on the 3D data.

Yet still further in carrying out the above objects and other objects ofthe present invention, a method is provided for developing dimensionalinformation about an array of objects, each of the objects including asurface. The method includes the steps of obtaining a first set of datarepresenting maximum specular reflections from the surfaces of theobjects, computing height estimate data for the array of objectsutilizing the first set of data and analyzing the height estimate datato obtain an estimate of the height.

In further carrying out the above objects and other objects of thepresent invention, a method is provided for measuring at least onedimension of an interconnect on a specular wafer. The method includesthe step of measuring the wafer at three or more non-colinear locationsto obtain reference data. The method includes further includes forming areference plane from the reference data. The method also includes thestep of scanning the wafer to obtain scan data based on the referenceplane. The method finally includes the step of determining the at leastone dimension of the interconnect based on the scan data.

The above objects and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a ball bump with an interconnectingwire;

FIG. 2 is a top schematic view of a 3D image of leads and defects in atray using only triangulation and illustrating an error caused by traywall reflection;

FIG. 3 is a side schematic view of a specular solder ball on a wafer andillustrating the ball's imaging geometry;

FIG. 4a is a top schematic triangulation-based image of a number ofsolder balls on a shiny “dummy” wafer utilizing triangulation forobtaining Z measurement, bump presence information and defectinformation;

FIG. 4b is a top schematic confocal slice image, similar to FIG. 4a,utilizing a confocal channel to obtain diameter information, Zmeasurement and defect information;

FIG. 5 is a side schematic view of a solder ball on a pad mounted on asubstrate and illustrating the filtering action of a spatial filter suchas a confocal slit;

FIG. 6 is a top schematic view similar to that of FIG. 2 after spatialfiltering with best focus near the nominal lead and burr locations usingthe confocal channel;

FIG. 7 is a side view of a ball bump on a pad illuminated from the topby laser light and illustrating side lobes outside the point ofinterest;

FIG. 8 is a schematic view of a confocal subsystem constructed inaccordance with the present invention including control logic to controllow and high N.A. laser beams;

FIG. 9 is a schematic view of a simplified combined triangulation andconfocal system constructed in accordance with the present inventionwithout optical elements;

FIG. 10 is a schematic view of a combined triangulation and confocalsystem constructed in accordance with the present invention;

FIG. 11 is a top schematic view of a semiconductor die enlarged fromFIG. 12 with spherical mirror balls mounted thereon;

FIG. 12 is a top schematic view of a wafer having a plurality of dies tobe inspected;

FIG. 13 is a schematic side view, partially broken away, of a sphericalmirror (i.e. solder ball) formed on a solder pad and its correspondingconfocal slice as viewed from the top of the mirror;

FIG. 14 is a block diagram flow chart illustrating an exemplary methodfor measuring microscopic targets;

FIG. 15 is a side schematic view of a solder ball on a wafer with awaist of a laser beam at the approximate midway point of the ball;

FIG. 16a is a top view of an image of a solder ball obtained throughtriangulation and processed to obtain peak information to avoidcrosstalk;

FIG. 16b is a top view of an image of the solder ball obtained throughthe confocal channel and processed to obtain diameter (i.e. edge)information;

FIG. 17 is a side view of a solder ball on a board illustrating expectedtip and center locations as well as confocal slice locations; and

FIG. 18 are graphs of normalized detector power versus height (i.e. beamwaist at Z=0) with Gaussian propagation off spherical and flat mirrorswith the innermost solid-line graph being of a diffuse surface andwherein the graphs are not necessarily symmetric about the origin andmay have varying shapes resulting from the different physical propertiesof the surface.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 9 is a simplified schematic view, without optical elements, of anintegrated triangulation-confocal system, generally indicated at 10,constructed in accordance with the present invention. The system 10includes a laser 11 (i.e. L1), a beam deflector 12 and a beam splitterassembly 13. The system 10 also includes a pair of spatial filters inthe form of slits 14 and 15. The system 10 further includes first andsecond position sensitive detectors 16 and 17, respectively, and aphotodiode detector 18. The detectors 16 and 17 provide triangulationanalog signals to a triangulation signal processor 19 for triangulationsignal processing and the detector 18 provides confocal analog signalsto a confocal signal processor 21 for confocal signal processing. Theresulting digital Z (i.e. height) and grey scale data from the processor19 is combined with digital confocal slice data (≧12 bits typical) fromthe processor 21 by a data processor 23 which provides multi-sensor dataprocessing under the control of a computer controller 25 to obtaindefect and dimensional data for the object being inspected by the system10.

FIG. 10 shows a schematic representation, together with opticalelements, of an integrated triangulation-confocal system, generallyindicated at 110, constructed in accordance with the present invention.The system 110 generally includes a triangulation-based laser scanner112. The triangulation-based laser scanner 112 has optical elements(i.e. pre-optics), a beam deflector, and a laser transmitter. Theoperation of triangulation-based laser scanners is taught in the art.For example, U.S. Pat. No. 5,024,529 shows a preferred method for highspeed, triangulation-based imaging.

The system 110 also includes a transmitter beam expansion subsystem 114and a transmitter focusing subsystem 115. The system 110 furtherincludes a pair of receivers, each of which includes an opticalsubsystem 116 and a position sensitive detector 118 which are positionedat angles with respect to a laser beam incident on an object 120 whichmay have a spherical surface. Each optical-subsystem 116 preferablyincludes an anamorphic optical assembly to delivery energy to its smallarea detector 118 whereby speed, field of view, and signal-to-noiseratio are maximized.

The multiple detectors 118 are preferably used to improve the accuracyof triangulation-based measurements by transforming height and intensitydata based on confidence measures as taught in U.S. Pat. No. 5,546,189.Based upon specifications for inspection and knowledge of the objectstructure, either data channel can be used, or a knowledge-basedalgorithm can be used on a data processor 121 under control of acomputer controller 123 to merge the resulting z and grey scale leveldata. Efficient scanning methods to exploit regular or repetitivepatterns such as balls or bumps 200 formed on a die 202 as illustratedin FIG. 11 are known and taught in the art. For example, the symmetry ofrow/column arrangements can be exploited as taught in U.S. Pat. No.5,617,209. Such regular arrangements are found in the semiconductorindustry and others. Each ball 200 is typically 10-300 μm in diameter.The die 200 is an enlarged part of a wafer 204 shown in FIG. 12. Thewafer 204 is typically 6 inches in diameter.

Referring again to FIG. 10, a basic confocal subsystem is integratedwith the above-described triangulation-based scanning subsystem to offercombined triangulation and confocal capability. A confocal channel isadded with the addition of a beam splitter assembly 122, a collectionand delivery optical subsystem 124, a photodiode detector/amplifier 126,and preferably an “on-board” high resolution analog-to-digital converter128 (i.e., 12-16 bits).

A spatial filter having a relatively wide slit 130 reduces laser andoptical system spatial noise and provides the dual function of aconfocal slit to filter back-scattered light displaced from the plane ofbest focus and secondary reflections from outside the narrowinstantaneous field of view. Preferably, a second spatial filter havinga relatively narrow slit 132 is located between the subsystem 124 andthe detector/amplifier 126. This allows for improvements or adjustmentsin sensitivity without the risk of introducing diffraction effects intothe transmitted beam (i.e. the incident beam). The separate path isoptionally provided by the narrow slit 132 if increased sensitivity isrequired. In addition, the use of additional slits and detectors (ormultiple laser beams) provides an option for variable sensitivity toreduce possible tradeoffs in measurement time.

From a practical standpoint, it is desirable that the dimensions of theslits 14 (i.e. FIG. 9) and 130 be about 3-5 times a diffraction-limitedspot diameter. Finer discrimination is possible with the addition of atracking mechanism, perhaps with use of a piezoelectric controller tocompensate for any drift with time or temperature. The use of a singleslit is an option, but is not an essential element of the invention.

A narrow slit width is not always a crucial feature of the inventionbecause a defocus function can often be sampled at coarser intervals ofdepth and interpolation or curve fitting methods used to estimate theheight at the point of interest. If spatial filtering of the incidentbeam is not warranted and hence is not implemented, a single confocalslit 15 and 132 may be used in the separate beam path. In any case, if avery narrow slit is used, active stabilization may be needed to avoid anincrease in intensity noise induced by drift, microphonics, or otherexternal influences. The above embodiment allows for high definitionconfocal imaging without disturbing the triangulation image withundesirable diffraction effects.

The assembly 122 preferably includes a cube beam splitter which servesthe dual purpose of providing optical isolation to prevent feedbacklight from entering the laser cavity, and a relay system to deliverlight to the high speed detector 126. The best delivery system will bedesigned to uniformly fill the detector 126 so as to avoid errorsassociated with intensity variations along the detector surface.

Neutral density filters or LCDs can be used for static or dynamicexposure control. If an acousto-optic beam deflector is used, opticalattenuation can be controlled with RF drive power. Fast modulationmethods may also be implemented with either acousto-optic modulators orelectro-optic modulators. The latter is preferably implemented in awaveguide structure for maximum speed, compactness, and minimal powerdissipation. However, advancements in A-D converter technology haveresulted in 14-16 bit converters which operate at video speeds orgreater. These devices, along with advancements in memory speed anddensity, may often eliminate the requirement of exposure control. Suchtechnology, along with-wide dynamic range detectors, is preferred.However, the directional reflectance of many industrial objects spansabout 4 decades, so exposure control may be required in some cases forsufficient noise margin.

Subsequent to A-D conversion by the converter 128, an interface isneeded to acquire the multiple channels of data. For instance, aconfocal PCI-based image acquisition scan buffer or commerciallyavailable frame grabber or a confocal image memory 134 is provided. Thememory 134 is connected to the data processor 121 for processing itsoutput with the Z and grey outputs of a signal processor 135 whichprocesses the output of the detectors 118 as described below.

Continued advancements in high speed digital data transmission methodscan be used to minimize the number of components. For example,commercially available A-D boards providing 12 bits at 30 MHz data ratesare available presently, like the Compuscope Series from Gage AppliedSciences. Such advancements are advantageous because the triangulation(with single or dual detectors) data and the confocal image can bereadily acquired and stored in parallel, resulting in perfect temporaland spatial registration for “fusion” operations in the processor 121.Other alternative methods are known to those skilled in the artincluding “fire-wire” technology for digital transmission to PC memory.

In any case, it is clear that future revisions of hardware will not be astumbling block because memory is cheaper and will require less space,and processors will be able to handle multiple imaging modes. Indeed, itis possible to store several slices of confocal data for subsequentpointwise or volumetric filtering operations, increasing robustness andrejecting noise associated with peak detection (i.e. sorting) methods.Memory savings can result, for instance, by providing confocal data fromregions of interest which may be a small fraction of the total image.For example, a ball grid array measurement system may use confocalimaging for localized peak detection over a region of 16 pixels×16pixels.

As illustrated in FIGS. 3, 4 a, 4 b, and 10, in a preferred embodimentwhere the numerical aperture (N.A.) of the triangulation and confocalsubsystems are substantially matched, a single source of illumination isprojected and scanned, preferably with a single high speed, solid statedeflector, onto the object 120, which may be a solder ball 140 formed ona solder pad 142 which, in turn, is formed on a polished semiconductorwafer 144 as illustrated in FIG. 13. The resulting reflected light isthen sampled by the detector system at the separate detectors 118synchronously. Often, the incident beam will be substantially normal tothe object's surface as illustrated in FIGS. 9 and 10.

Alternatively, separate imaging units may be used with sequential dataacquisition and the data registered via software within the processor121 with the disadvantage of extra processing time and additionalcalibration to compensate for temporal or spatial misregistration. Thisadditional embodiment is not necessarily preferred, but may beacceptable if the system figure of merit is improved. In any case, asolid state deflector has an advantage of random access and high speedwindowing and is recognized herein to be advantageous for the high speedconfocal-based focusing system described herein.

Certain advantages may be achieved with the use of acousto-opticdeflectors, particularly if the access time is fast, corresponding totens of micro-seconds maximum. Alternatively, the use of electro-opticdeflection technology, preferably in the form of a sequence ofelectrically activated gratings embedded in a thin film structure,offers potentially exceptional performance, with access times of a fewnanoseconds and resolution of several hundred spots. The low delay ofthese deflectors will provide substantial improvements for inspection ofobjects in a predetermined or regular arrangement. An example of such adeflector is described in U.S. Pat. No. 4,902,088 (assigned to APAOptics). Micromirror technology may also be employed, provided thataccess times are fast enough to meet inspection requirements.

Alternatively, acousto-optic deflectors can be advantageous in certainapplications with an appropriate compromise between resolution(time-bandwidth product), acoustic velocity (delay), and scan angle.Line rates well beyond video are achievable. For example, TeO₂(Tellurium Dioxide) deflectors operated in longitudinal mode may provideaccess time on the order of 1 microsecond or less, with 32 or more spots(resolution). The line rates achievable with such a device areextraordinary when applied to localized region-of-interest dataacquisition. In certain line scan systems, the limit could become themotion of the translation mechanism used for the part or imaging unit.In some cases, two-dimensional deflection may be preferred to avoidbottle-necks, perhaps with a second acousto-optic device or low inertiamirror. In some cases, it may be advantageous to provide the fastscanning action with a second deflector and laser source confined to theconfocal subsystem.

It is instructive to illustrate one of the many novel exemplaryoperational modes of the multi-sensor system 110 of the presentinvention. As illustrated in FIG. 11, a mirrored semiconductor die 202with “spherical” mirror balls 200 mounted thereon, is scanned. In afirst pass, for example, the triangulation-based system will acquiredata from the wafer 202 and the balls 200 which partially represents thesurface profile. The data is sufficient to rapidly identify defectiveregions, including missing or defective bumps (i.e. at 42 and 44,respectively, in FIG. 4a), or certain surface defects. For surfaces withdiffuse reflection (as illustrated at 40 in FIG. 4a), the triangulationdata may be sufficient for height and diameter measurement.

As shown in FIG. 3, the displacement of points corresponding to specularreflection on a perfect mirrored spherical surface measurable withtriangulation from the peak of the ball 29 corresponds to only about a 1μm height offset for a typical ball with 150 μm diameter with atriangulation angle 2A of 30°. If, for example, the initial height ofthe scanning device is chosen such that the surface intersects with thewaist of the illumination beam, there is a possibility of rapid mergingof the data for peak height and diameter measurement.

Referring to FIG. 4b, simultaneously, data acquired along the confocalchannel, can be used to obtain a high contrast image of a ball 34, fromwhich the diameter is estimated. This estimation is superior to that ofthe triangulation system and camera-based systems; the data is devoid ofbackground noise due to the arrangement illustrated in schematic form inFIG. 5. Often the ball 34 will be specular providing large signals forthe confocal channel at the peak location. Furthermore, the highcontrast image can be used to locate surface defects which may not bevisible in the triangulation data because of occlusion or low signalslimited by the diffuse reflection coefficient.

If, for instance, the imaging system 112 is positioned so that the beamwaist is at the approximate 50% height level of the ball (nominally),edge definition is maximized, and the contrast will remain high. Thisfavorable condition occurs because of the extreme range of objectdirectional reflectance, spanning several decades. The wafer“mirror-like” return, although reduced well below a maximum, providesfor good contrast. The simplified sketch of FIGS. 4b and 13 shows thetype of image which is expected from a “spherical mirror” on a flatspecular background using a single slice from the confocal channel, nearbest focus.

As previously mentioned, FIG. 1 shows a solder ball 20 formed on asolder pad 24. A tip 22 of the ball 20 necks down. Specific tradeoffsbetween depth of focus, object height and background reflectance willinfluence the contrast.

Similarly, FIGS. 2 and 6 show the resulting images from a single passwith leads near best focus. Likewise, defects on a wafer whichcorrespond to defects in traces, extra material and missing material canbe detected.

It has been determined that the standard deviation and absolute accuracyof triangulation-based height measurements is larger than desirable oncurved, specular surfaces because of optical crosstalk inherent intriangulation systems necessarily having limited data, as illustrated inFIGS. 3, 7 and 16 a. Optical crosstalk will be manifested in thetriangulation channel by a localized “contrast reversal” (FIGS. 3, 7 and16 a). In the regions near the peaks shown in FIG. 16a, higher portionsof the solder ball will appear lower and vice versa. Hence, the peakdetection shown in FIG. 16a is important to the triangulationmeasurement on such surfaces.

Accuracy can be enhanced and further verification of the correctgeometry can be done using the triangulation-based height estimate (theposition sensitive measurement corresponding to FIG. 16a), diameterestimate (i.e. FIG. 16b), and lateral location of the intensity maximumto specify the subsequent confocal slices, perhaps in conjunction withhigh speed “windowing” or region of interest scanning for substantialimprovements in speed. These slices may be obtained, for example, withrapid translation of optical elements in the scanner over a narrowrange, allowing for high speed.

Referring now to FIG. 14, there is illustrated in block diagram, flowchart form an exemplary data collection and processing method of thepresent invention.

At block 300, reference data is acquired from a wafer 301 of FIG. 15 ora wafer 38 of FIGS. 4a and 4 b. Typically, the confocal channel is usedto generate this data if the wafer 301 or 38 is specular or unpatterned.

At block 302, the sensor is positioned at a nominal predeterminedlocation, for example, so that the waist of a laser beam 303 is at thenominal expected center 305 of a solder ball 307 which is a sphericalmirror.

At block 304, triangulation 3D data is acquired as well as a confocalslice in a first pass. Such image data is illustrated in FIGS. 4a and 4b for triangulation data and confocal data, respectively, for bothspecular spheres and for cylinders having diffuse reflection.

At block 306, the data is analyzed with the processor 121. For aspecular object, the 3D triangulation height data is analyzed byisolating the peak information to avoid crosstalk, as illustrated inFIG. 16a. A height estimate is obtained. The confocal diameter and peakposition is obtained by analyzing the data from the confocal slice asillustrated in FIG. 16b.

At block 308, both the height estimate and the diameter are comparedwith expected results and/or specifications including expectedconsistency between confocal and triangulation data as illustrated inFIG. 17 as well as sphericity (i.e. expected conformation to a sphere).

At block 310, additional data, such as confocal slices and/ortriangulation data, is acquired for selected regions of interest of theobject as also illustrated in FIG. 17.

It is instructive to compare and contrast conventional confocalmicroscopy with the preferred method utilized herein. Conventionalmethods use a narrow slit or pinhole, low f/# (high numerical aperture)optics, and small increments for z axis positioning of the object orsensor. Although these principles can be advantageous in carrying outthe invention described herein, the requirements for measurement of manymicroscopic objects can be met with fewer slices at coarse increments.For example, when the system is utilized to measure the depth ofreflective objects including solder balls, wafers, traces, conductiveepoxy and copper, measured intensity changes continuously with depth,but the optical medium is homogeneous (i.e., air).

The data processing algorithms of the present invention may beimplemented in special purpose hardware or in software within theprocessor 121 and can be applied to either “featureless” surfaces (i.e.,a mirror) or rough objects. The slices may be acquired by translatingthe part or imaging head (conventional) or, alternatively, a pluralityof detectors could be used each with a diaphragm (slit or pinhole) asshown in the above-noted USSR patent document and U.S. Pat. No.5,448,359.

Experimental confocal data and simulations indicate that the variationin sensitivity (change in intensity per unit change in depth) isstrongly dependent upon the object structure and reflectancecharacteristics. Profiles of curved specular objects having differentradii, plane mirrors, diffuse opaque surfaces, and translucent objectsshow significant variation. The peak intensity, half width, andasymmetry are variable (see FIG. 18). The curved specular surfaces havefocusing power dependent upon the curvature, and produce measurablechanges in intensity for relatively small lateral displacements of thespot position on the surface.

The beam propagation characteristics, directional sensitivity, andincreased intensity noise produce adverse conditions for measurement.This is in contrast to imaging a diffuse reflector which is “wellbehaved”. The typical assumption of a least squares quadratic fit astaught in the prior art may often be oversimplified, and inadequate fora description of all types of signatures (profiles representingintensity changes with depth). As a result, generalized curve fitting orprediction methods may include “weights” or other adjustments based uponinformation about the surface.

Curve fitting and peak estimation methods based upon prior informationare preferred to estimate the peak location for improved noise immunity.A preferred method of data processing utilizes non-linear and linearfiltering for spike removal and smoothing, respectively. This approach,known from the art of image processing, tends to maximize the fidelityof the profile without excessive smoothing. Such a linear filteringalgorithm may be implemented with a linear convolution kernel, butpreferably will be an adaptive smoothing method. Such algorithms are nowcommonplace in data processing packages like MathCad 6.0.

Yet another alternative for data acquisition is to use a combination ofa predetermined range of translation with a known spacing of a pluralityof sensors. The number of detectors and their spacing along the opticalaxis can be traded off with mechanical height adjustment for a specificmeasurement speed requirement. For example, a piezoelectric actuatorcould be used to provide rapid translation over a narrow axial range, orthe relationship of optical elements in the transmitter system,including the slit(s) or pinhole(s) changed so as to vary the effectiveoptical path length.

Those versed in the art of confocal imaging will recognize thattradeoffs between measurement speed, optical power, accuracy, cost, andsensor compactness can be analyzed to select an appropriate balancebetween a plurality of detectors and axial motion with positionfeedback. Because objects to be imaged with this invention typicallyproduce wide dynamic range requirements, the preference is to maintainhigh optical efficiency with losses, which result from beam-splitting,minimized. For example, the peak of a solder ball can then be isolatedand measured as previously described.

The method and system of the present invention preferably includes aconfocal arrangement, but does not exclude the addition of an additionaldetector in the optically collinear path, which receives collinear lightto produce an intensity image without an associated spatial filter. Thissecond detector, which is not spatially filtered, may be used for grayscale measurements and for comparison of the relative intensity ofattenuated light with the maximum return from the object 120. Thisinformation could be useful for gray scale based measurements, guidingthe search process or normalizing the confocal image relative to anintensity maximum if curve fitting is done. This coaxial energy could becollected in any number of ways.

In another mode of operation, absolute height measurements of one of thesolder balls 200 of FIG. 11, relative to the bare or patterned die 202of the wafer 204 can be done using focusing methods taught in the art.When properly fixtured, the dies 202 do not exhibit much warpage.Measuring the die 202 at a few locations (i.e. as indicated by the “X”sin the four corner locations of FIG. 11) using planar surface predictionshould be adequate. The four corner locations can be measured using suchlocal fiducials if available, or the bare wafer surface can be measuredusing the confocal channel. Once again, the imaging head may bepositioned along the Z axis to find the general location, and theintegrated optical system translated rapidly.

If the scanner 112 includes an acousto-optic deflector or other solidstate deflector, for example, an acquisition speed for a region can beminimized by limiting the scan FOV and, for instance, restricting thescan to the X axis only (no Y axis motion for focus measurements mode),quickly moving the Z axis, and recording the intensity. Alternatively, a“ramp” can be generated if the wafer 202 is assumed to be flat over aFOV. In either case, the Z axis location should be recorded as afunction of time for best measurement capability.

Fully utilizing the high speed scanner windowing capability availablewith a beam deflector (say 64 pixels), and the use of a plurality ofpoints for fitting the defocus function would improve reliability whilemaximizing speed. Speed is the limiting factor for most measurementsystems and the method described here would minimize impact formeasurement of “featureless” surfaces.

The axial motion may be divided into large range for coarse location ofobjects, and high speed narrow range operation for measurement. In theformer case, the imaging head or part is translated. In the latter case,optical elements or groups may be translated using any of the methodsknown in the art provided that the proper relationships between thescene, objective lens, and confocal spatial filter are maintained. Forexample, inspection of microelectronic assemblies might require about0.25″ for coarse location, but an active measurement range of only0.004″ for a low f/# (high numerical aperture) transmitter beam, forinstance in the range of f/2 to f/6. The latter motion could be inducedwith, for example, a piezoelectric translation stage or similaractuator. Fast motion could mandate an increase in the requireddeflection speed of the scanner 112, leading to an overall advancementfor confocal imaging in general. If a deflector with nanosecond responsetime is available, then the speed will be limited by the axial motionmechanism.

Similarly, of great advantage would be a subsystem to translate thefocus position along the optical axis which does not utilize movingparts, or at least only requires miniature, high speed dynamicassemblies. Acousto-optic deflectors, for instance, can change theireffective focal length in the scan direction by applying a non-linear,variable frequency waveform. This electrical/acoustical effect is knownas the cylindrical lens effect. The device also introduces deflectionwhich, for instance, could be compensated with a high speed, smallamplitude deflector, perhaps a micro-mirror. Future advancements in themicro-mechanical technology and integrated optics may lead todevelopment of high speed focus translation methodologies.

Many additional modes of operation can be derived and will be understoodby those skilled in the art. For instance, it may sometimes be desirablefor the N.A. of the triangulation subsystem be relatively low, providingfor good edge contrast over a large depth of field. On the other hand,the highest confocal resolution may be desired. The issue can beaddressed by using time, spatial, or wavelength multiplexing and a pairof collinear beams of different wavelength or diameter (f/h). Asillustrated in FIG. 8, such a pair of beams introduces dual lateralresolution operation for both confocal and triangulation subsystems.

As illustrated in FIG. 8, a confocal subsystem of an integratedtriangulation confocal system is generally indicated at 210. The system210 includes control logic 212 which controls, by multiplexing, a pairof laser sources 214 and 216. Expanders 218 and 220 expand the laserbeams emitted by the laser sources 214 and 216, respectively. A mirror222 reflects the expanded beams from the expander 218 to obtain acentral ray which is combined at beam combiner 224 with the expandedbeam from the expander 220.

The system 210 also includes a beam deflector 226 which deflects thecombined low N.A. beam and the high N.A. beam to a beam splitter 228, alens 230 and to an object 232. The resulting beams reflected from theobject 232 then are passed through the lens 230, reflected by the beamsplitter 228, spatially filtered by a single or multiple slit 234,focused by a lens 236 and detected by a detector 238. Alternatively, thelaser sources 214 and 216 could have different wavelengths.

In yet another embodiment of the present invention, multiple slits withvarying dimension could be used to provide variable sensitivity anddepth of field. This option requires additional optical elements(including detectors) and can be readily implemented by those skilled inthe art.

If a solid state deflector is used, either acousto-optics orelectro-optics diffraction gratings, with wavelength multiplexing, thenadditional optics will be needed to expand the scan width according tothe wavelength ratio. With wavelength multiplexing, spectral filters canbe used to provide discrimination and eliminate crosstalk.

Time multiplexing would preferably be implemented with two lasers whichare pulsed in sequence. Then, either the confocal or triangulationchannels or both are read. The advantage is dual lateral resolutionoperation for both the confocal and triangulation modes.

Those skilled in the art will recognize the versatility of theinvention, and extensions and operational principles which are withinthe spirit of the invention. The feature of multiple beams or slits in amultiplexed system, either beam being available for use in thetriangulation and confocal channels, provides a choice of expanded ornarrow depth of field, which can be advantageous for measurement ofobjects with an extended depth range. For example, the high N.A. channelmay be used to locate defects in thin conductive, dielectric layers, orprovide contrast improvement for surface inspection, while the widerrange is useful for examining interconnects.

This foregoing description shows illustrative embodiments and principleof operation but should not be regarded as restrictive. The versatilityof measurement methods and systems is a direct benefit of the inventionwhich is limited only by the following claims.

What is claimed is:
 1. A method for developing dimensional informationabout an array of objects, each of the objects having a surface, themethod comprising the steps of: obtaining a first set of datarepresenting maximum specular reflections from the surfaces of theobjects in the array of objects; computing height estimate data for thearray of objects utilizing the first set of data; analyzing the heightestimate data to obtain an estimate of the height; and obtainingadditional information about the array of objects using a confocalsensor system based upon the estimate of the height, wherein theconfocal sensor system has a spatial filter and a confocal detector toproduce sufficient confocal slices of the objects to obtain confocaldefect and dimensional information of the objects.
 2. The method asclaimed in claim 1 further comprising the steps of obtaining a secondset of data represented by a region in proximity to the maximum specularreflection and analyzing the second set of data by peak location toreduce optical crosstalk.
 3. The method of claim 1 wherein each of theobjects has a diameter and wherein the dimensional information is adiameter of at least one of the objects.
 4. The method of claim 1wherein each of the surfaces is a shiny curved surface which is are-flowed, substantially spherical, solder ball surface.